This invention relates to coating compositions, their preparation and use.
Coatings used for the original painting of motor vehicles and for their repair are required to have very good physical properties such as hardness and resistance to water and solvents. The coating compositions are also required to have good application and appearance properties so that they are easy to apply to a substrate using spray application and result in final films having good gloss.
One class of coating compositions which is known for use in painting motor vehicles comprises a hydroxyl functional polymer, such as a polyester or acrylic polymer, and a polyisocyanate. These two components react together after application to the substrate to form a polyurethane coating. These compositions are often thus referred to as xe2x80x98polyurethane coating compositionsxe2x80x99 although strictly the polyurethane is only formed when the coating hardens.
These polyurethane coating compositions have been widely used in the vehicle refinish industry for many years but suffer from the disadvantage that polyisocyanates are known respiratory sensitisers and so considerable care must be taken in their use, particularly to prevent inhalation of the paint spray by the operator during spraying. This usually requires the operator to wear an air-fed breathing mask. Clearly, an alternative type of paint is desirable.
We have now discovered a surprising alternative class of coating compositions which can give application and final film properties which are equal to or better than those of polyurethane coatings. These alternative coating compositions comprise a polymer incorporating uretdione groups and acrylate, maleic or fumaric groups, which polymer of cross linked with a polyamine cross linker. This represents a total departure from the last 25 years of developments in refinish coating compositions which have virtually all focused on polyurethane compositions.
GB-1409496 and U.S. Pat. No. 5,814,689 disclose uretdione containing polyurethanes cross linked with polyamines. There is however no reference to the use of polyesters or mixed polyesters having acrylate, maleic or fumaric groups.
EP-424745 discloses a maleic or fumaric containing polyurethane polyester for UV of four radical core. There is however no mention of amine cross-linking.
According to the present invention there is provided a coating composition comprising:
i) a polymer having uretdione groups, and
ii) a polyamine cross linker,
said polymer being a polyester, or a mixed polyester-polyurethane, which polyester or mixed polyester-polyurethane has other groups capable of reacting with the polyamine cross linker, which other groups are acrylate, maleic or fumaric groups.
The uretdione groups can be pendant, but preferably they are in the polymer backbone. The polymer is preferably a polyester.
Suitable polyesters are derived from a polybasic acid, a polyhydroxy compound and a compound having uretdione groups and reactive groups.
Polybasic acids are compounds having two or more carboxylic acid groups. Such polybasic acids are well known in the polyester art. Examples of suitable polybasic acids are C1-6 alkane dioic acids such as succinic acid, glutaric acid, adipic acid or hexanedioic acid, cycloaliphatic acids such as hexahydrophthalic acid, unsaturated alkane dioic acids such as fumaric or maleic acids, dimer acids, and aromatic acids such as phthalic acid, isophthalic acid and trimellitic acid. Ester-forming derivatives of such acids can also be used in place of the free acids. Ester-forming derivatives include anhydrides and lower alkyl, for example methyl or ethyl, esters. Mixtures of two or more acids or their ester-forming derivatives can be used.
The polyester may have other amine-reactive groups capable of, reacting with the polyamine crosslinker such as acetoacetate or epoxy. More preferably, the polyester has units of formula (I); 
in which the carbonyl groups can be cis or trans. These units also react with amine groups on the polyamine crosslinker and provide additional sites for crosslinking of the polyester. These units can be derived from maleic acid or fumaric acid, or from ester-forming derivatives of these acids such as maleic anhydride and the lower alkyl, for example methyl or ethyl, esters of these acids. Preferably, these units are derived from maleic acid or maleic anhydride.
Preferably at least 25% of the groups on the polymer which will react with the polyisocyanate crosslinker are uretdione groups.
Polyhydroxy compounds are compounds having two or more hydroxyl groups and are well known in the polyester art Examples of suitable polyhydroxy compounds are diols such as ethylene glycol, propylene glycol, 1,3-propane diol, butylene glycol, 1,4-butane diol, 1,6-hexane diol, neopentyl glycol, triols such as trimethylol propane, and glycerol, tetrols such as pentaeryatritol and higher polyols such as sorbitol. Mixtures of two or more of these polyols can be used.
Uretdione groups are groups of formula (I)I; 
in which R and Rxe2x80x2 are the residues of the polyester polymer backbone.
These uretdione groups can be incorporated into the polyester backbone by use of a compound having uretdione groups and also reactive groups which will react with the other components of the polyester so as to become incorporated into the polyester backbone. Examples of reactive groups are hydroxyl groups, acid groups, epoxy groups, and isocyanate groups. One example of a compound having uretdione groups and reactive groups is the uretdione of a diisocyanate, such as hexamethylene diisocyanate uretdione (available as HD100(trademark) which also comprises a proportion of higher adducts, from Arco) of Formula (III); 
In this case the reactive groups are isocyanate groups which will react with hydroxyl groups on the polyols and so become incorporated into the backbone of the polyester.
Mixed polyester-polyurethanes can be made in a similar way to polyeters but using a di- or polyisocyanate instead of some of the polybasic acid. Suitable diisocyanates are tetramethylxylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate and toluene diisocyanate. Suitable polyisocyanates include the isocyanurate trimers, allophanates and uretdiones of diisocyanates such as those described above as well as the reaction products of these diisocyanates with polyols. Polyols are compounds having two or more hydroxyl groups. Suitable polyols include trimethylol propane, glycerol and pentaeryatritol. Many such polyisocyanates are commercially available, for example under the Desmodur trade mark from Bayer or the Tolonate trade mark from Rhodia.
Preferably the polyester has 0.5 to 4 mols/kg uretdione groups based on the weight of the polyester. Preferably the polyester has a number average molecular weight of 600 to 4000, more preferably 600 to 2000.
The polyamnine is a compound with two or more amine groups per molecule, preferably 2 to 6. Preferred diamines, having two amine groups, are aliphatic and cycloaliphatic diamines. Examples of suitable aliphatic and cycloaliphatic diamines are ethylene diamine, propylene diamine, 1,6-hexanediamine and cycloaliphatic diamines of the formulas; 
Also useful are polyamines containing three or more primary amine groups. Examples of such polyamines include tris-(2-aminoethyl) amine, N(CH2CH2NH2)3 and the products of the Jeffamine(trademark) series manufactured by Jefferson Chemical Company having the general structure 
where A is the residue of an aliphatic triol, such as glycerol or trimethylol propane, and x, y and z are integers, the sum of which is between 5 and 85. Suitable compounds containing three or more amine groups can be made by joining together compounds with fewer amine groups. For example diethylene triamine can be converted to its bis diketimiine by reaction with methyl diisobutyl ketone and then the remaining secondary amine group can be reacted with a diisocyanate or a diepoxide to give a polyamine having four amine groups or with a triisocyanate to give a polyiamine having six amine groups. Diamines can be reacted with triisocyanates to give triamines. Diamines can be reacted with a maleate ester such as dimethyl maleate to give a triamine.
The amine groups can be free amine groups but preferably the amine groups are blocked primary amine groups. Examples of blocked primary amine groups are aldimines and ketimine groups.
Aldimines or ketimine groups are groups of formula (IV) 
in which R1 and R2 are independently H, alkyl, cycloaliphatic or substituted alkyl, aryl or cycloaliphatic group.
Preferred ketimines and aldimines are obtained by reaction of a polyamine with an aldehyde or ketone. Examples of aldehydes and ketones suitable for use in making the aldimines and ketimines are those containing 1 to 8 carbon atoms such as propionaldehyde, butyraldehyde, isobutyraldehyde, acetone, methyl ethyl ketone, methyl isobutyl ketone, diisopropyl ketone, cyclopentanone and cyclohexanone.
One particularly preferred aldimine is Desmophen TPLS 2142(trademark) from Bayer and one particularly preferred ketimine is VPLS 2965(trademark) from Bayer.
The components are preferably used such that the ratio of the amine groups on the polyamine to the number of amine-reactive groups in the polyester is in the range 0.7:1 to 3:1, more preferably 0.9:1 to 1.5:1.
The polyester, or polyester-polyurethane can be made by conventional means. Generally the components of the polyester are melted together or dissolved in a suitable solvent, such as xylene. The melt or solution is then heated so as to remove the water produced in the reaction between the acid and the hydroxyl groups. When the components are melted together, the water can conveniently be removed using a fractionating column at temperatures of between 150 and 250xc2x0 C. When the components are dissolved in a solvent the water can conveniently be removed by azeotropic distillation using a Dean and Stark apparatus at the reflux temperature of the solvent A combination of these method can be used, initially melting the components together and removing water using a fractionating column and then adding solvent and removing filter water using a Dean and Stark apparatus. If a polyisocyanate is to be included, either to introduce the uretdione groups or to produce a polyurethane or polyester-polyurethane, then this is generally added after reaction of the other components, and at a lower temperature, such as between 50 and 100xc2x0 C. because the isocyanate groups are much more reactive than acid or ester groups.Preferably a catalyst for the isocyanate-hydroxyl reaction is included such as a tin catalyst, for example dibutyl tin dilaurate.
Preferably the compositions also comprise a volatile organic solvent. The volatile organic solvent can be any solvent which will dissolve the polyisocyanate, and the hydroxyl functional polymer. It can be an aliphatic or aromatic hydrocarbon such as Solvesso 100(trademark), toluene or xylene, an alcohol such as butanol or isopropanol, an ester such as butyl acetate or ethyl acetate, a ketone such as acetone, methyl isobutyl ketone or methyl ethyl ketone, an ether, an ether-alcohol or an ether-ester or a mixture of any of these
The compositions can also contain other conventional paint additives such as, pigments, fillers, UV absorbers and flow aids.
The compositions can be made by mixing the components. When the composition comprises an organic solvent, the compositions can be made by dissolving the components in the organic solvent in any order. The composition is preferably made by adding the polyamine to the other components shortly before use. This avoids problems with the potlife of these compositions.
The coating composition of the invention can be applied to the surface of a substrate and then allowed or caused to dry and cure. According to the present invention there is provided a process for coating a substrate which comprises the steps of applying a layer of a coating composition according to the present invention to a surface of the substrate and thereafter causing or allowing the layer to cure.
The coating composition can be applied by conventional means such as by brush, roller or spray, preferably spray. The substrate can be for example, metal, plastic, wood or glass and can be a previously painted surface. The compositions are particularly useful for refinishing motor vehicles and especially as primers.
The applied layer can be allowed to cure at ambient temperature in the case where the hydroxyl polymer and crosslinker react together at ambient temperatures. Alternatively the layer can be baked at elevated temperatures, for example 50-120xc2x0 C. to accelerate curing. Drying and curing typically takes between 5 minutes and 24 hours depending on the ambient conditions and on the particular components used. Conveniently it takes about 15 minutes to about 5 hours.
According to the present invention there is also provided a coated article obtainable by the process.